1. Field of Invention
This invention relates in general to removing scratches from plastic surfaces, and in particular to a method and means which reconstitutes compact discs damaged due to scratches and other surface imperfections.
2. Description of Prior Art
This invention relates to producing smooth surfaces on substrates of plastic materials. In particular it provides a method for repairing damage due to scratches or other imperfections on the surface of plastic storage media for digitally encoded information of the type commonly embodied in so-called Compact Discs or CDs. Compact discs or CDs as they are commonly referred to, are optically encoded digital storage media. The physical structure of the disc is composed of a polycarbonate plastic known under the trade name of Lexan TM, a product of General Electric Corporation. Fabrication of a CD is accomplished by injecting molten polycarbonate into a cavity where micro pits are formed which represent encoded information. These micro pits are arranged in a spiral data track. The side where the micro pit track has been formed is coated with a thin layer of deposited metal, usually aluminum or gold, to make the surface reflective. Subsequent treatment involves adding a lacquer coating over the delicate reflective surface as a protective layer followed by a silk screened label.
The polycarbonate serves both as a support structure and as an optical window through which a laser is focused on the reflective layer containing the data tracks. Each pit in the track varies in length according to the encoded digital data. Depending on whether the laser light is scattered or reflected back is related to pit presence and length and provides the basis for recovering a digital data stream which represents the desired recorded information.
Anything which interferes with the ability of the laser scanning system to focus on the encoded pits can lead to data loss. Scratches, debris and even oil deposits from finger prints all have potential to cause such data interruption. To minimize the effect of minor laser obstruction, CD playback systems typically employ an electronic data interpretation algorithm which can recover lost information.
Despite error correction schemes, ordinary handling of compact discs can lead to minor or severe mechanical surface damage as a result of contact with abrasive particulate matter or by simply allowing one CD to be placed in sliding physical contact with another CD. Accumulation of this damage can exceed the error correction capabilities of the CD reader hardware and software, especially if a scratch or combination of scratches occurs in a curvilinear rather than a radial orientation. Due to the sequential nature of the encoded data, such damage can render vast amounts of information unavailable for recovery, ultimately rendering the optical disc useless. The net effect is a costly loss of data such as in CD ROM library databanks. The only recourse left to the user faced with such damage is the purchase of a replacement disc, if available.
Since the compact disc is fabricated from plastic, means used to polish plastic have been applied in an effort to remove scratches and other surface imperfections. There has been a variety of attempts to develop procedures that will remove scratches and other mechanical surface defects from CDs. Many of the proposed approaches involve polishing with fine abrasives dispersed in a lubricant to remove surface irregularities, sometimes with a deformable material to fill in unwanted scratches or pits.
For example, U.S. Pat. Nos. 5,334,335, 5,337,524 and 5,407,615 issued to W. C. Norville, disclose various formulations of abrasives, solvents, fillers and lubricants designed to abrade surface projections and fill in pits and scratches in plastic surfaces including compact discs. However, it is difficult to produce a smooth surface finish on polymeric materials by abrasion, in part because they are too soft and flexible to abrade readily and in part because they are not good thermal conductors and therefore do not dissipate frictional heat very effectively. The resulting temperature rise can soften the plastic and result in gumming and poor finish control, as discussed in Modern Plastics Encyclopedia, pp. 533-536, S. Gross, Editor-in-Chief (McGraw-Hill, New York, 1974). Also to be remembered in the case of CDs is that removal of material from the surface of the disc by abrasion or polishing can cause permanent local variations in disc thickness. Thickness irregularities can result in focus error and data loss. Furthermore, any filling of pits and scratches can also cause data loss due to possible laser scatter at the transitional interface between the plastic media and a filler agent.
U.S. Pat. Nos. 4,654,917 and 4,783,870 to C. K. Yeung disclose an apparatus that combines rotation of a CD with radial motion of a pad so that the entire surface of the disk can be evenly rubbed by the pad, after it is soaked with a cleaning solution, to remove fingerprints, oil deposits, and dust particulates or other debris. Neither disclosure offers a means to rectify the deleterious effects of scratches or other disc surface damage.
International Application WO 95/21890 by M. J. Smithlin, published on Aug. 17, 1995 describes a composition and a method for repairing a CD. The composition comprises a mixture of a wax with a refractive index which approximates that of CD polycarbonate, a solvent for the wax, some fine abrasive material and an emulsifier to keep the abrasive in suspension. The solvent must be one that does not soften or dissolve the disc material. A small amount of the mixture in paste or liquid form is applied to the disc surface and gently rubbed with a soft cloth or other applicator until the defect is smoothed and filled. The disc is considered repaired when examination with a microscope indicates that the edges of the scratch are beveled or rounded and the scratch is filled with the composition. It should be noted that this method does not remove particulate matter produced as the result of a scratch or other surface damage or take into account that surface damage often has light scattering effects not addressed by a filler agent. Further, this method must be considered as a temporary repair only since such filler can be dislodged.
Several investigators have taken a different approach in which the plastic is softened in a controllable manner so that surface tension forces can reduce the surface area, thereby smoothing surface projections and scratches. D. Duchane in U.S. Pat. No. 4,376,751 and in an article in Materials Engineering, vol. 9, No. 4, p 68 (1981) decribes a method in which the object with a plastic surface is immersed in a solution of solvent in a non-solvent diluent. Over a period of hours the concentration of solvent in non-solvent diluent is slowly decreased to zero. This procedure is intricate, lengthy, and consumes a substantial amount of solvent per treatment. Further, a significant amount of nonsolvent may be permanently incorporated into the polymeric article, an undesirable result in the case of a CD. The process assumes that a slow removal of solvent is necessary to prevent recurrence of the surface defects. Indeed, Duchane has asserted that "methods in which solvent is allowed to evaporate from a plastic surface are so fast that surface wrinkling occurs." (Chemical Engineering, Nov. 25, 1985, page 15.)
U.S. Pat. Nos. 4,260,873 to M. R. Simmonds and 5,448,638 to Edmonds both describe a method in which heated solvent vapor partially dissolves a plastic surface, softening it sufficiently so that surface tension forces will tend to decrease the surface area and thus smooth the plastic surface. A fairly complex and expensive apparatus is required and it is difficult to control the extent of vapor penetration by manipulating the vapor pressure, temperature and duration of exposure, to attempt to preclude crazing and stress cracking.
U.S. Pat. No. 4,302,418 to Cullis discloses a process where a fluid medium is used for polishing plastic surfaces made of a material which is insoluble or inert within that fluid medium unless subjected to a temperature where plastic deformation would occur. The process includes employing a solvent vapor at an elevated pressure to liquefy a plastic surface without causing total plastic component deformation. This method requires complex temperature and pressure regulation and can result in stress crazing of plastic media or could cause a transparent plastic media to become opaque. Further, use of solvent vapor will render a plastic surface susceptible to contamination by dust or particulate matter which can become permanently embedded on the suface of the plastic media being treated. Finally, the required apparatus is expensive and the process is time consuming.
Examination of the prior art as set forth in the cited references indicates that all of the previously proposed methods for producing smooth surfaces on plastic materials comprise fairly slow and complex procedures and are not particularly effective. By contrast the method of the present invention achieves the same objectives by a procedure that is simple, rapid, uses minimal amounts of reagents and requires no special apparatus.